Developing VR games without a head mounted display (HMD) of your own is possible, however, it may not be the best way to go about it. To get the most out of the VR experience, it is recommended that you use an HMD, as it is the device that allows you to immerse yourself in the virtual world.
That being said, if you do not have an HMD of your own, there are some options available to help you develop an engaging VR experience. Firstly, if you plan to develop a PC-based game, you can take advantage of desktop VR support, meaning you are able to connect your PC to an external display via a USB adapter, allowing you to play your VR game on the external display instead of an HMD. To do this, you will need a VR-ready gaming PC with a powerful graphics card and a compatible USB adapter, such as the Oculus Rift, HTC Vive, or Windows Mixed Reality.
Another option to develop a VR game without an HMD of your own is to use a game-development platform, such as Unity or Unreal Engine. These platforms are designed to allow developers of all skill levels to create virtual experiences, either on a PC or mobile device. By taking advantage of the game-development tools available through the software, you can develop immersive VR experiences without needing specialized hardware.
Finally, to experience the VR game you are developing without an HMD, you can track your progress via 3D software such as Blender or Maya. By taking a screenshot of the environment or a short video of the interaction, you can see firsthand how your design comes to life.
Overall, there are options available for developing a VR game without an HMD, however, for the best experience it is recommended that you invest in an HMD of your own.